Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials, particularly eyeglass lenses, are specifically required to have, as physical properties, low specific gravity, high transparency and low yellowness, high heat resistance, high strength and the like, and as optical properties, high refractive index and high Abbe number. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. However, as the refractive index is increased, the Abbe number is decreased. Therefore, it has been studied to improve both of the refractive index and the Abbe number. Among methods which have been proposed, the most representative method is a method using an episulfide compound as described in Patent Document 1.
This episulfide compound is obtained by producing a sulfur-containing epoxy compound in which a sulfur atom at the epithio moiety is replaced with an oxygen atom and then sulfidating the compound. As methods for producing a sulfur-containing epoxy compound, the production methods described in Patent Documents 1-3 have been proposed, and the yield and transparency of resin obtained by curing an episulfide compound obtained by sulfidation have been improved.
However, in these production methods, hydrogen sulfide is used as the main raw material and it is difficult to handle it. Patent Document 1 shows a production method in which hydrogen sulfide is not used. However, according to the method, the yield was low, and since a large amount of oligomer was produced, it was difficult to perform purification by distillation after that.
Accordingly, the development of a method for producing a sulfur-containing epoxy compound, wherein hydrogen sulfide is not used and the production of oligomer is suppressed, has been desired.
Further, it is necessary to cause a reaction with a basic compound after the reaction, and therefore a two-stage reaction is carried out. Accordingly, the development of a method for producing a sulfur-containing epoxy compound in good yield in one step, wherein hydrogen sulfide is not used and the production of oligomer is suppressed, has been also desired.
In addition, as a technique of increasing the refractive index, the episulfide compound having disulfide shown in Patent Document 4 has been proposed. This episulfide compound is obtained by producing a sulfur-containing epoxy compound in which a sulfur atom at the epithio moiety is replaced with an oxygen atom and then sulfidating the compound.
However, in this production method, hydrogen sulfide is used as the main raw material and it may be difficult to handle it. Moreover, it is necessary to synthesize a disulfide bond using halogen after the reaction, and the reaction is complicated.
Furthermore, when trying to further increase the refractive index, disulfide is the limit of the technique shown in Patent Document 4, and it is impossible to produce an epoxy compound having polysulfide that is tri- or higher sulfide. Therefore, a method for producing an epoxy compound having polysulfide has been desired.
Accordingly, the development of a method for producing an epoxy compound having polysulfide, wherein hydrogen sulfide is not used, has been also desired.